1. Field of the Invention
The invention relates to a polishing apparatus for polishing a work piece, for example the end edge of a metal ring for a power transmission belt employed in a belt type continuously variable transmission.
2. Description of the Related Art
A vehicle is typically installed with an automatic transmission that adjusts a transmission gear ratio in accordance with a traveling condition of the vehicle. A belt type continuously variable transmission (CVT) that adjusts the gear ratio steplessly may be installed as the automatic transmission.
A CVT is capable of transmitting engine output efficiently and achieves improvements in fuel efficiency and traveling performance. The CVT uses a power transmission belt and a pair of pulleys, for example, to realize stepless shifts continuously by varying an effective diameter of the pulleys hydraulically. The power transmission belt is looped around an input side pulley attached to an input shaft and an output side pulley attached to an output shaft. By varying a groove width of the input side pulley and output side pulley, a loop radius of the power transmission belt relative to the input side pulley and output side pulley varies, and as a result, a rotation speed ratio between the input shaft and the output shaft, or in other words the gear ratio, can be varied continuously and steplessly.
The power transmission belt is formed by preparing a plurality of elements with varying thickness and passing stacked metal rings through the plurality of elements. A high degree of dimensional precision is required in the metal rings of the power transmission belt. More specifically, the metal ring is formed by cutting a cylindrical drum, which is formed by welding the end portions of super-strength steel thin plates together, into predetermined widths. Following cutting, the end edge of the metal ring is sharp, and therefore finishing is required to polish the end edge into a highly precise curved shape.
A polishing apparatus of a related art field for polishing an end edge of a metal ring will now be described on the basis of FIGS. 9 to 11 and with reference to FIG. 1. As shown in FIG. 1, a polishing device 50 includes a ring rotation device (not shown) that holds a metal ring 2 and rotates the metal ring 2 in a circumferential direction, a columnar polishing brush 3 that polishes an end edge 22 of the metal ring 2 by bringing bristle tips thereof into contact with the end edge 12 as the bristle tips pass the end edge 12, a holding plate 4 on which a plurality of the polishing brushes 3 stand upright at intervals in a circumferential direction and which holds one end portion of each polishing brush 3, and a support plate 6 having through holes 5 into which the polishing brushes 3 are respectively inserted, for adjusting a bristle length of the polishing brushes 3 extending from the through holes 5 and suppressing splaying of the polishing brushes 3. Note that the polishing apparatus 50 according to the related art field is provided with two polishing brush units 10, each of which is constituted by the polishing brushes 3, the holding plate 4, and the support plate 6, and these two polishing brush units 10 are disposed in close proximity.
The polishing brush 3 is formed by bundling together a plurality of bristles constituted by resin wires containing a polishing material into a columnar shape. One end portion of each polishing brush 3 is fixed to an outer peripheral portion of the holding plate 4 such that the polishing brushes 3 are disposed at intervals in a circumferential direction. The holding plate 4 is formed in a disc shape, and a rotary shaft (not shown) is provided in the center of the holding plate 4. Further, a motor (not shown) is connected to the rotary shaft so that the holding plate 4 can be driven to rotate by driving the motor. As a result, when the holding plate 4 is rotated, the polishing brushes 3 revolve.
As shown in FIGS. 1 and 9, the support plate 6 is formed in a disc shape having a substantially identical diameter to the holding plate 4. The through holes 5 into which the polishing brushes 3 are respectively inserted are formed on an outer peripheral portion of the support plate 6 in positions corresponding to the polishing brushes 3. Note that in FIG. 1, eight polishing brushes 3 are provided, whereas in FIG. 9, sixteen through holes 5 are formed in the support plate 6 for the polishing brushes 3. The reason for this difference is that FIG. 1 is a schematic diagram, and in actuality, the number of polishing brushes 3 matches the number of through holes 5 in the support plate 6. Further, as shown in FIG. 9, a large-diameter insertion hole 11 formed in the center of the support plate 6 and a plurality of (four in FIG. 9) small-diameter insertion holes 12 formed around the large-diameter insertion hole 11 are provided for fixing an elevator rod. Furthermore, a plurality of (eight in FIG. 9) insertion holes 13 are formed on the inside of the through holes 5 for the polishing brushes 3 at intervals in a circumferential direction to fix a plurality of guide rods. When the elevator rod is driven, the support plate 6 is capable of moving in the axial direction of the polishing brushes 3 and can be fixed in a predetermined position.
To polish the end edge 22 of the metal ring 2 using the polishing apparatus 50 according to the related art field, first, the metal ring 2 is attached to the ring rotation device such that the metal ring 2 straddles a part of the two polishing brush units 10, 10 and the end edge 12 thereof opposes the bristle tips of the polishing brushes 3. At the same time, the support plate 6 of each polishing brush unit 10 is moved to the bristle tip side of the polishing brushes 3 and fixed into position such that the bristle tips of the polishing brushes 3 project from the respective through holes 5 in the support plate 6 by approximately 5 mm to 20 mm. Note that the end edge 22 of the metal ring 2 and the bristle tips of the polishing brush 3 are disposed with an overlap by approximately 1 mm. The ring rotation device and the holding plate 4 are then driven to rotate, whereby the metal ring 2 rotates in a circumferential direction (a direction indicated by an arrow in FIG. 1: a counter-clockwise direction), and the respective holding plates 4 of the polishing brush units 10 rotate (in a direction indicated by an arrow in FIG. 1: a clockwise direction) such that the polishing brushes 3 revolve. Thus, as shown in FIG. 11B, the bristle tips of each polishing brush 3 contact the end edge 22 of the metal ring 2 while passing the end edge 12, and as a result, the end edge 22 of the metal ring 2 is polished to a predetermined curvature.
As shown in FIGS. 10 and 11, however, when the end edge 22 of the metal ring 2 is polished by the polishing brushes 3 of this polishing apparatus 50, the bristle tips of the polishing brush 3 deform so as to splay to the left and right sides of an advancement direction (the sites indicated by broken lines in FIG. 10), and also deform diagonally rearward in the advancement direction (the site indicated by a broken line in FIG. 11). When the polishing process is continued with the bristle tips of the polishing brush 3 in this deformed state, stress is concentrated in a single site of the bristle tips, leading to partial wear and a reduction in the durability of the polishing brush 3. Moreover, when the bristle tips of the polishing brush 3 deform such that partial wear occurs, a contact area between the bristle tips and the end edge 22 of the metal ring 2 deviates from the contact area at the start of the polishing, and as a result, an R shape of the end edge 22 of the metal ring 2 becomes unstable. Furthermore, when the bristle tips of the polishing brush 3 deform so as to splay, a phenomenon whereby the bent bristle tips strike a side face and so on of the metal ring 2 occurs, and as a result, this part is damaged, leading to a reduction in the strength of the metal ring 2.
In Japanese Patent Application Publication No. 2005-254339 (JP-A-2005-254339), which serves as related art proposed to solve the problems described above, a polishing apparatus includes: a ring holding portion for holding a metal ring in a circular form such that one end edge thereof is exposed; a ring rotating portion for rotating the metal ring in a circumferential direction via the ring holding portion; a brush holding portion for holding a polishing brush formed by bundling together a plurality of wires into a columnar shape so that the polishing brush can rotate using a longitudinal direction of a bristle base of the polishing brush as an axis; and a brush moving portion for moving the polishing brush to traverse a rotary track of the metal ring held by the ring holding portion via the brush holding portion and polishing the metal ring by bringing the polishing brush into contact with one end edge of the metal ring. When the metal ring is polished by the polishing apparatus, the polishing brush spins in accompaniment with the rotation of the metal ring upon contact with the metal ring, and therefore the part of the polishing brush that contacts the metal ring is always different. As a result, partial wear of the polishing brush can be reduced, and curling of the bristles can be prevented.
With the invention of JP-A-2005-254339, an attempt is made to reduce partial wear of the polishing brush by making the polishing brush capable of spinning. However, during polishing, the bristle tips of the polishing brush bend away from the metal ring, and therefore deformation of the polishing brush cannot be suppressed. Hence, the basic problem is not solved.